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POPULAll HISTORY OF LICHENS. 



placed in the Arctic and Antarctic regions by another species, 

 JJsnea melaxantha, which is interesting in a threefold point 

 of view, but which may be considered a variety of U. bar- 

 bata peculiar to a polar climate. It is one of the most 

 handsome and arborescent Lichens, closely assimilating them 

 to the Plianerogamia. Its cross section exhibits a structure 

 resembling, on superficial examination, that of an exogenous 

 stem, having a distinct axis and a separable cortical layer ; 

 and it is the only saxicolous species, for in the Falkland 

 Islands, Dr. Hooker says, it covers "the surface of the 

 quartz rocks with a miniature forest, seeking the most ex- 

 posed situations, and there attaining its greatest size and 

 beauty." Some varieties of U. harbata have a more limited 

 geographical range than others : var. f orida, for instance, 

 is inferior in this respect to var. plicata ; this is to be ex- 

 pected, when we consider that these varieties must depend 

 greatly on differences in habitat and climate. What we 

 now regard as varieties were by the older authors considered 

 distinct species ; but they are frequently found graduating 

 into each other in such a way, that it is impossible to de- 

 termine under which form or name to arrange them, and 

 several of them may be met with growing in the same 

 forest, nay on the same tree. Of all the forms the most 



